Several types of level sensors are currently used to detect a height of an ink in an ink supply reservoir in printers. The word “printer” as used in this documents means any apparatus, such as digital copier, bookmaking machine, facsimile machine, multi-function machine, etc. which produces an ink image on media for any purpose. Examples of the level sensors include: (1) conductive level sense probes; (2) over-driven thermistors; (3) vibrating beams; and (4) optical sensors.
Known optical sensors include a photoemitter and photodetector positioned at one end of a housing and an optical prism at the other end of the housing. Such a sensor is shown in FIG. 5A. The sensor 300 has a base 304 and a housing 308 that terminates in a glass prism 312. The glass prism has a conical shape for purposes noted below. A photoemitter 316, such as an LED or other coherent light source, is positioned in the base 304. A photodetector 320 is also positioned in the base at a position that is approximately adjacent to the photoemitter. A non-reflective liquid level 324 is also shown in the figure. When the photoemitter is activated to generate a beam of light as shown in the figure, the conical prism totally internally reflects (TIR) the light on a path that is parallel to the emitted light beam so it strikes the photodetector, which generates an electrical signal that is inversely proportional to the intensity of the light striking the photodetector. When the prism 312 of the sensor 300 is immersed in a non-reflective liquid as shown in FIG. 5B, the differences in the coefficients of refraction at the interface of the prism and the non-reflective liquid enable most of the emitted light beam to pass through the prism and into the liquid. Thus, the photodetector 320 generates an electrical signal that has a voltage that is greater than the one produced when the sensor is not immersed in non-reflective fluid. Therefore, the response of the photodetector 320 indicates whether the sensor 300 detects the non-reflective liquid or not. Placing the sensor 300 at positions corresponding to levels where an ink supply needs replenishment, a controller can operate a pump or the like to send ink to the reservoir in which the sensor 300 is located.
A problem arises when the sensor 300 is positioned within an ink reservoir that contains a reflective ink, such as a white colored ink. In such a reservoir, the reflective ink directs ambient light reflected from the ink into the photodetector when the ink covers the prism. Consequently, the photodetector still generates an electrical signal that approximately the same level as the one it generates when the reflective ink level is below the prism. Thus, the sensor 300 is inoperative in reflective ink. Being able to obtain the advantages of optical level sensors in reflective fluids would be useful.